[en] Bunch compression technique is considered which is supposed to be done before the beam transfer from the U-70 to be UNK. The latter is supposed to operate in the (0.4x3) TeV beam colliding mode. The technique includes the mismatching of the beam prior to its rotation in phase space around the stable fixed point. In addition to microwave instability two main problems are treated in detail - the nonlinearity of the synchrotron motion and the interaction of the beam with the RF cavities during the compression. Possible cures are investigated such as using an harmonic cavity at double of the RF frequency in order to linearise the main voltage. Also the suppression of the 1-st harmonic of the beam current is shown to be necessary for successful compression of the bunch. The influence of higher-order harmonic of the induced voltage is evaluated. 6 refs.; 9 figs.; 2 tabs